The present invention relates to a drive control method, namely a method for controlling a heavy duty drive, in particular a heavy duty drive for a vertical roller mill for comminuting brittle materials such as cement raw material, and to a corresponding drive system operating according to said method.
Vertical roller mills of the above mentioned type, having a grinding table rotating about the vertical and grinding rollers above the grinding table, are subject to severe mechanical vibrations. The resulting forces and torques can be so powerful that the grinding process has to be stopped in order to prevent damage to the drive train, namely the electric motor and gearbox in particular, or to the plant as a whole.
In order to minimize such vibrations, the mill operator has hitherto had to design the process parameters, i.e. in particular the contact pressure of the grinding rollers, composition of the material to be ground and amounts of grinding additives, such that the vibrations excited remain below a critical level. However, this means undesirable process design limitations which negatively impact many areas. These include the range of products that can be made from the respective ground material obtained, the effectiveness of the mill, the energy input required and the cost-efficiency. Nevertheless, such measures are unreliable, as much experience is required for correct process control and the properties of the natural materials before and after grinding are necessarily always different. This makes it necessary to continuously optimize the process and adapt it to suit the raw material.
However, extreme vibration states—known in the industry as “rumbling” of the mill, repeatedly occur, with the result that the mill has to be stopped and then restarted. This is to the detriment of the availability and productivity of the plant. There is also the risk of gearbox and plant damage. To obviate this problem, process control has hitherto been organized particularly defensively in order to prevent these mill vibrations as far as possible. However, the production rate, product quality and the range of manufacturable products suffer as a result.
Against this background and because of the increasingly exacting requirements in respect of availability, efficiency and Total Cost of Ownership (TCO), the design and arrangement of the electrical and mechanical components of a drive system and of the respective drive train of a heavy duty drive, in particular of a vertical roller mill, are becoming increasingly important.
For vertical roller mills, drive systems comprising a gearbox and an electric motor in the form of an asynchronous motor, preferably a wound rotor, and a frequency converter feeding the electric motor constitute a preferred solution. Here the mill gearboxes are in practice implemented as variants of bevel or spur gear planetary mechanisms. The purpose of the gearing arrangement is not only speed and torque conversion but also to absorb the axial grinding forces and transfer them into the base.
Controlling such a drive system for a vertical roller mill essentially presents the following problems in practice:
In order to be able to ensure optimum process control, the first, apparently trivial task of the drive is to deliver the predefined rotation speed of the grinding table. As the process torque demand at the grinding table fluctuates, speed control is required.
The load fluctuations and vibration excitations acting on the drive mechanism are influenced by impulsive loads such as those produced when the grinding rollers encounter coarse material, stochastic loads of the grinding process, periodic excitations from the gearbox and mill kinematics, and a varying contact pressure of the grinding rollers. The interaction of these load influences results in a complex load cycle which can even set off resonance vibrations.
In addition to the drive train vibrations, an unstable, i.e. fluidizing or undulating grinding bed, for example, can also cause extreme vibrational states of the mill, in particular mill rumbling.
Lastly the grinding of natural products makes it largely unpredictable how the grinding process must be adjusted in order to guarantee quiet running of the mill. It is therefore always a challenge for the operator at the control desk to find the correct process parameters. In the end, although the drive alone can quieten a poorly adjusted process, it cannot correct it.